Double-make contact switching apparatus with improved alternating current arc suppression means



Nov. 10, 1970 C. F. CASSON A DOUBLE-MAKE CONTACT SWITCHING APPARATUS WITH IMPROVED ALTERNATING CURRENT ARC SUPPRESSION MEANS Filed Oct. 10, 1968 LOAD - INVENTOR. CHARLES F. CA5 0A1 AGEN T United States Patent U.S. Cl. 219-501 6 Claims ABSTRACT OF THE DISCLOSURE A circuit utilizing a double-make (double-break) switch to connect a load to an AC. source is provided with a protective loop having a bilateral semiconductor triode connected across the switch with the triode gate connected to the switch bar through a resistance.

This invention relates to electrical switching means of the double-make contact type for selectively coupling a load to an alternating current source, and more particularly to such switching means including arc-suppression means effecting improved contact life and current ratings for such switching means.

Mechanical switches are presently made which are capable of conducting relatively high continuous currents, but they are severely limited in their ability to survive the effects of breaking such currents. This limitation is particularly severe at voltages above the characteristic arc voltage of any specific contact material used for the contacts. To increase contact life under such adverse conditions has usually involved increasing the mass of the contacts to absorb the are energy and making the contacts of a precious metal.

It is well known that when contacts separate during intended normal switching, harmful arcing usually occurs therebetween. Also, after contacts first engage during switch closing, they may repeatedly bounce (commonly called chatter) which is attended by arcing until final engagement is attained. Such arcing accelerates contact consumption by pitting and burning which contributes to relatively short contact life and reduces the reliability of the overall circuit.

Heretofore, attempts to reduce harmful contact arcing having involved mechanical means reducing the amount of contact bounce such as biasing springs, cam systems, or the like, which increase switch size and cost, and introduce reliability problems into the overall switch circuitry. In recent years, various solid state devices such as transistors, controlled rectifiers, AC switches, and the like, have been used in an attempt to provide more durable switches and to increase circuit reliability. While some solid stateswitch devices are capable of switching currents on the order of ten or more times greater than they can continuously conduct and can accomplish such switching without the arcing associated with mechanical switches, such devices have certain limitatons which make them less than completely satisfactory for switching and conducting of high currents. Some of these limitations are relatively low continuous current conducting capability, susceptibility to damage by sustained current and voltage transients, and the need for complex associated circuitry such as biasing networks, triggering networks, and the like.

Accordingly, an object of this invention is to provide an electrical switching apparatus having improved switch contact protection, which obviates problems heretofore associated with attempts to increase switch contact life.

. Another objeect of this invention is to provide a new and novel means for sensing the presence of an arc to be suppressed and effecting suppression of said are in a switch apparatus of the double-make contact type.

Another object of this invention is to provide a switching apparatus of the double-make contact type in which substantially all arcing resulting from contact disengagement is eliminated, thereby increasing contact life and the current ratings of such switching apparatus.

Another object is to provide a switching apparatus of the type described which is of rugged and uncomplicated construction, is relatively economical to manufacture, and provides increased reliability of the overall switch circuitry.

A further object is to devise such a switching apparatus employing a solid state device and in which no external triggering circuit is needed.

Another object is to provide a switching unit of the type described for connecting an alternating current power source to a load, wherein the magnitude of currents switched is greater than that possible for contacts alone, and wherein the magnitude of currents continuously conducted is greater than that possible for solid state switch devices alone. A related object is to provide such as switching unit which uses a minimum mass of contact material.

This invention contemplates the protection of electrical switch contacts in double-make contact type switches by the use of a monostable quiescently non-conductive, bilateral semiconductor device having a first electrode connected to one of the double-make switch contacts and an AC power source; a second electrode connected to the other of the double-make switch contacts and a load; and a third electrode, of a gate or trigger control type, connected through conductive means to a third switch contact, the latter comprising the common switch bar or shunt engaging the double-make contacts of the switch.

When the contacts disengage due to normal intended disengagement or to contact bounce after engagement, the device is trigered substantially instantaneously to the conductive state by the voltage conditions at the contacts acting on the gate terminal of the bilateral semiconductor device. The current in the circuit following the disengagement of the contacts, is conducted through the device until the contacts reengage or the normal cycle of current being conducted drops below the holding current required to maintain the device in its conducting state. With the switching aparatus of this invention, continuous conduction is provided by the contacts, and conduction during switching disengagement and bounce is provided by the semiconductor device. This permits a combination of high-current capability, for both switching and continuous conducting, heretofore not available.

The foregoing and other objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawings wherein a single embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration purposes only and are not to be construed as defining the limits of the invention.

The drawing is a schematic diagram of a switching means of the double-make contact type including arc suppressive and protective means of the present invention.

Referring in detail to the drawing, an electric switching means 10 is shown as comprising a double-make contact type switch S and a bilateral semiconductor switch means D.

The switch S comprises first and second in-line contacts 12 and 14 shunted selectively by a third contact or switch bar 16. The first contact 12 is connected through a power lead P1 to one side of an AC source 18 such as a 120 volt, 60 cycle source; and the second contact 14 is connected through a conductive lead P2, load L and power lead P3 to the other side of the AC source 18.

The bilateral semiconductor D includes first and second power terminals, 20 and 22, connected, respectively, to the same circuit nodes as the first and second switch contacts 12 and 14 of the switch S, thereby placing the power transmitting path of the semiconductor D in parallel with that of the switch S. The semiconductor D further includes a gate terminal 24 which is electrically connected through a coupling resistance R to the contact bar 16 of the switch S.

In the switch S, the contact bar 16 moves to make or break the contacts 12 and 14 substantially simultaneously, Thus, the switch S is said to be of the double-make contact type and is to be construed as representative of all types of electric switch structures wherein a double-make or double-break contact action occurs.

The bilateral semiconductor D is a solid state switching device which will conduct current through its power terminals 20 and 22 in either direction and can be rendered conductive by a small gate signal applied to the gate electrode 24 or by the voltage across the power terminals exceeding the rated breakover voltage in either direction of conduction. Once rendered conductive, the bilateral semiconductor D will remain in the conductive state until the current flow through the power terminals 20 and 22 thereof drops below a minimum holding value. In AC applications, such a cut-off condition is effected on every half-cycle when the current reverses or if a shunt is placed across the power terminals 20 and 22. The gate terminal 24 does not effect any cut-off function and is only utilized as a means for eifecting the conductive state in the semiconductor D at a desired point in time.

Therefore, the bilateral semiconductor D is in actuality a static AC switch of a type generically known in the art as Triac, a gated, bi-directional thyristor device.

The time required to trigger or gate the semiconductor D to its ON state is relatively short, i.e., on the order of a few micro-seconds. Further, such gating can be accomplished by an applied voltage of either positive or negative polarity at the gate terminal 24.

Therefore, for the purpose of protecting mechanical switch contacts, the bilateral semiconductor D acts with such a high relative rate of speed to the contacts 12, 14 and 16 of the switch S as to be substantially an instantaneously responsive device for all practical purposes.

Both the mechanical switch S and the bilateral semiconductor D can be included in an integral structure or modular unit switching means with only two external connections, namely, a first common terminal 26 for the first switch contact 12 and first semiconductor terminal and a second common terminal 28 for the second switch contact 14 and the second semiconductor terminal 22.

Then, by merely placing the common terminals 26 and 28 in series with the AC source 18 and the load L, the switching means 10 is fully operable to control the energization of the load L in a self-protected, double-make contact configuration.

In operation, assuming that the switch bar 16 of the switch S is engaged with the first and second switch contacts 12 and 14 and the load L is thereby energized by the AC source 18, the bilateral semiconductor D is shunted through the switch S across the common terminals 26 and 28 of the switching means 10 and is in a nonconductive or quiescent (OFF) state.

Now, if the switch bar 16 is caused to break simultaneously with the first and second contacts 12 and 14 of the switch S, arcs are formed between the bar 16 and first contact 12 and the bar 16 and second contact 14.

Upon the formation of these arcs, the switch bar 16 becomes the center tap of a voltage divider comprised of the two are discharge paths which create a potential 4 drop across the first and second common terminals 26 and 28.

The voltage appearing at the switch bar 16 is reference either to the first common terminal 26 and first semiconductor terminal 20 or to the second common terminal 28 and second semiconductor terminal 22 for gating purposes depending upon which half-cycle of alternating voltage is present at the time the contacts 12 and 14 were caused to break with the switch bar 16.

Regardless of the polarity of the gate potential, however, a gating current is effected through the coupling resistor R and gate terminal 24 and the bilateral semiconductor D is gated ON, effecting a closed shunt path around the switch S and quenching the arcs occasioned by breaking the contacts of the switch S.

At the end of the immediate half-cycle from the AC source 18, the bilateral semiconductor D is turned OFF due to the reversal or zero cross-over of the current which drops below that value which sustains conduction in the semiconductor D.

The same instantaneous arc suppression and attendant protection of the contacts of the switch S in the switching means 10 is effected during the making of the contacts 12 and 14 with the switch bar 16 should contact bounce occur and generate arcs between said switch bar and contacts.

Since a half cycle of a 60-cycle AC wave consumes approximately 8.3 milliseconds, the bilateral semiconductor device D may be rendered conductive for a time period in the range of something greater than zero seconds to 8.3 milliseconds, depending upon the point in the current wave at which the bilateral device D is rendered conductive. The relative time period of particular concern, when the contacts of the switch S are to be brought into engagement from the open position, is the bounce time which is measured from the moment such contacts commence to reopen after initial engagement until all contact bounce or chatter has ceased, the bounce time sometimes consuming several milliseconds.

The are voltage appearing across the contacts of the switch S has a greater magnitude and higher dv/dt characteristic than does the AC wave from the source 12, resulting in the initation of the arc suppression function in the switching means 10 as previously described herein.

Typical circuit values for the foreging switching means 10 comprise the use of a coupling resistance R with a value of ohms (1 watt) and a bilateral semiconductor D comprised of a General Electric Triac type SC46B.

From the foregoing, it can be readily seen that the life of the contacts 12, 14 and 16 of the switch S in the switching means 10 and the switching current rating thereof are substantially increased when protected by the bilateral semiconductor D in accordance with the principles of this invention, since the semiconductor D is triggered instantaneously to the conductive (ON) state to shunt the contacts of the switch S before the arc voltages reach such a magnitude as to have adverse efiects on the said contacts. Because of this increased life and rating, the reliability of the switching means 10 of the present invention is markedly increased over that of the simple switch S.

Further, the simplicity and self-contained nature of the switching means 10 is readily apparent in that no external triggering or gating circuitry is needed to trigger the bilateral semiconductor D to a conductive (ON) state.

It is to be understood that the magnitude and frequency of the power source are not to be construed as limiting the invention, since switch devices of the type described are presently commercially available that will operate effectively with power sources of other magnitudes and over a wide range of frequencies.

Although but one embodiment of the invention has been illustrated and described in detail, it is to be expressly understood that the invention is not limited thereto. Various changes may be made in the design and arrangement of the parts without departing from the spirit and scope of the invention as the same will now be understood by those skilled in the art.

What is claimed is:

1. Switching means for selectively connecting a load with an alternating current source comprising:

switch means having first and second contact means arranged for relative movement into and out of engagement with one another; one of said contact means being adapted to provide an open circuit connection between a load and an alternating current source and the other of said contact means being adapted to effect a complete circuit connection between such a source and load through said one of said contact means; bilateral semiconductor means having first and second electrode means connected one to each side of said one of said contact means and third electrode means connected with said other of said contact means;

said bilateral semiconductor means having a normally non-conductive state and being triggerable to a conductive state in response to harmful electric arcing conditions across said first and second contact means to substantially instantaneously shunt the electric energy of said electric arcing around said contact means; and

gating circuit means including said first and second contact means, deriving an electric signal from an arcing condition across the said contact means and applying said signal to said third electrode means to substantially instantaneously render said bilateral semiconductor means conductive and shunt said contact means to extinguish said arcing condition.

2. The invention defined in claim 1, wherein said bilateral semiconductor means comprises a bi-directional, gated thyristor.

3. The invention defined in claim 1, wherein said gating circuit means further includes electrical resistance means in series with said other of said contact means and said third electrode means.

4. The invention defined in claim 1, wherein said one of said contact means comprises a spaced pair of double-make contacts and wherein said other of said contact means comprises a conductive switch bar adapted to substantially simultaneously make and break with said double-make contacts; and

said double-make contacts and said switch bar, during an arcing condition therebetween, comprising a voltage divider providing an input voltage to said gating circuit means from said switch bar.

5. The invention defined in claim 1, wherein said one of said contact means comprises a spaced pair of doublemake contacts and wherein said other of said contact means comprises a conductive switch bar adapted to substantially simultaneously make and break with said double-make contacts;

said double-make contacts and said switch bar, during an arcing condition therebetween, comprising a voltage divider providing an input voltage to said gating circuit means from said switch bar; and

wherein said gating circuit means further includes electrical resistance means electrically coupling said conductive switch bar with said third electrode means.

6. The invention defined in claim 1, wherein said one of said contact means comprises a spaced pair of doublemake contacts and wherein said other of said contact means comprises a conductive switch bar adapted to substantially simultaneously make and break with said double-make contacts;

said double make contacts and said switch bar, during an arcing condition therebetween, comprising a voltage divider providing an input voltage to said gating circuit means from said switch bar;

wherein said gating circuit means further includes electrical resistance means electrically coupling said conductive switch bar with said third electrode means; and

wherein said bilateral semiconductor means comprises a bi-directional thyristor triode.

References Cited UNITED STATES PATENTS 3,309,570 3/1967 Goldberg 31711 3,401,303 9/1968 Walker 317-11 3,408,538 10/1968 Gurwicz 317-11 3,475,620 10/ 1969 Murray et al. 317-11 BERNARD A. GlLI-IEANY, Primary Examiner F. E. BELL, Assistant Examiner US. Cl. X.R. .31711 

